Telescoping arms are useful in innumerable applications for holding objects at selectable positions. A telescoping arm typically has an outer sleeve with an inner rod coaxially positioned within. The inner rod slides so that an exposed portion of selectable length extends from the sleeve to provide a variable overall length. Generally, frictional forces between the rod and the sleeve hold the apparatus in the selected position. Sometimes, internal locking or braking mechanisms may be used for this purpose.
Typical telescoping arm assemblies require close manufacturing tolerances if they are to carry loads and have a smooth sliding action. While sliding rods and attached bushings may easily be manufactured to precise tolerances, the sleeve interior is more difficult to control dimensionally. An added manufacturing step may be required to drill the sleeve to form a precise bore; more complicated machining may be required if the bore is to have a non-circular cross-section.
As an alternative to precisely machined rigid components, conventional telescoping arms may include compliant or abradable bushings to accommodate low tolerance sleeves, such as those produced by an extrusion process. A compliant bushing for such an assembly typically has several compressible points that extend radially from the bushing in a star shape. The points are compressed or abraded as the bushing is installed in the sleeve. As the bushing transmits lateral loads, the star points are subject to further deformation, which results in a progressively loosening action over the life of the apparatus. The small load-bearing area of the bushing wears at an accelerated rate in any case.
The present invention provides a bushing for a telescoping arm having a rod coaxially received in a sleeve. The bushing is constructed with a rigid body portion that is closely but loosely received to slide within the sleeve. Flexible spring portions attached to the bushing body are biased against the sleeve interior to provide a controlled frictional resistance to sliding, and a resistance to lateral vibration under small loads. When there is a clearance gap between the bushing body and the sleeve, the springs flex in response to moderate loads until the body portion contacts the interior of the sleeve to bear larger loads. The substantially rigid body portion limits the range of spring flexing to prevent permanent deformation or damage to the springs.
The apparatus has an extended product life, over which the action and frictional resistance of the apparatus remains relatively constant due to the limited force applied to the spring contact points. In addition, because the springs are initially flexed to bias against the sleeve, minor surface wear at the spring contact points will not appreciably reduce the biasing force. As a result, the present invention has a low manufacturing cost and a consistently smooth action throughout its life.